1. Field of the Invention
The present invention relates to abrasive media, processes for using the abrasive media, and apparatus for practicing processes with the abrasive media. The media are thin flexible abrasive sheeting used for lapping, polishing, finishing or smoothing of workpiece surfaces. In particular, the present invention relates to such processes and apparatus that use removable or replaceable abrasive sheeting that are able to operate at high surface speeds, and apparatus that secures the abrasive sheeting to a support (particularly in an annular distribution of abrasive material on a face of the abrading platen). The support may optionally move the sheeting at those high speeds (preferably without the use of adhesive layers between the sheeting and the support). The apparatus, processes and abrasive media provide a high degree of control over the contact point or contact plane of the abrasive sheeting and the article that is to be lapped, polished, finished or smoothed.
2. Background of the Art
High speed lapping and grinding using fixed abrasive on sheet disks for both rough grinding and smooth polishing is now a practical reality. Most performance issues relate to two primary concerns, 1) hydroplaning caused by water lubricant and 2) vibrations created by grinding machine component dimensional inaccuracies and thickness variations of abrasive disks along their tangential surfaces. Unique answers for the first problem of hydroplaning have been defined, numerous solutions have been created and most of these solutions have been implemented or evaluated.
The most serious problem remaining is the availability of high quality abrasive article sheets that have certain important characteristics. The sheets should be of a sufficient dimension (e.g., at least a 6 inch (15.3 cm) diameter, at least a 12 inch (30.5 cm) diameter, or at least an 18 inch (45.7 cm) or larger diameter, and have islands comprising abrasive particles (preferably secured to a substrate and preferably arranged in an annular band). The particles have an uppermost abrasive surface that is extremely flat and of uniform thickness. Conventional flat surface grinding or lapping platens are set up to use the full surface area of a circular shaped flat flexible sheet of abrasive. However, the abrasive contact surface speed of the rotating disk varies from a maximum speed at the outer radius to zero at the innermost center at the disk (where the radius is zero). The grinding material removal rate is roughly proportional to the surface speed of the moving abrasive, so that most of the grinding or lapping action, and the most efficient grinding or lapping action occurs at the outer portion of a rotating disk. Not only is the inside portion of the abrasive disk not used to remove workpiece surface material, but also this portion of the abrasive is not worn down by the workpiece, resulting in a shallow, cone shape of the abrasive disk surface. This uneven wear continues with usage of the disk, with the cone angle progressively increasing to a sharper angle. This cone angle is translated to the surface of the workpiece that is intended for rigid axis lapping of a workpiece and prevents precision flatness grinding of the workpiece, transferring uneven surface contour to the workpiece surface. An effective answer to this uneven wear is to create an abrasive disk with a narrow annular band of abrasive material (at the outer edges of the annulus), allowing the abrasive to wear down more evenly across the full surface of the abrasive disk (which is essentially the annulus, not a continuous circular surface) as the disk is used. This type of media is not available commercially and probably would not be with present production methods. This is because the continuous method of manufacturing abrasive disks cannot technically or economically produce the necessary annular configuration. Presently, an important method of manufacturing a circular abrasive sheets is to coat a continuous web backing with diamond particles to form a coated sheet material and then to punch out round disks from the coated sheet material. Effectively, most of the expensive inner surface area of these disks is wasted. If a conventional coated disk is used with a platen having an outer raised annular ring, then all of the abrasive coated area located at a radius inside the ring is not used as it does not contact the workpiece surface.
Furthermore, it is not practical to punch out radial rings from a coated web sheet for a number of reasons. First, there is not necessarily a ready market for the smaller disk that remains left over from the center punch-out for the annular ring. Also, there is a large waste of coated web material left over between the circular disks that are cut out, even with proficient xe2x80x9cnestingxe2x80x9d of the circular rings. Furthermore, the annular ring of coated abrasives made of thin 0.005 inch (0.127 mm) thick polyester web has limited structural body strength for handling and mounting so that it cannot be practically used on a platen without creating many problems, including the problem that water and grinding swarf tend to collect under the inside edge of the loose annular ring sheet. Furthermore, round or bar raised-abrasive islands having a thin top coating of expensive diamond particles are needed to compensate for hydroplaning affects at high surface speed lapping. The only island type of abrasive media now available which can reduce hydroplaning is a diamond particle metal plated Flexible Diamond Products abrasive sheet supplied by the 3M Company (Minnesota Mining and Manufacturing Co.). However, due to the manufacturing process of this product, the product is commercially limited by at least two counts. First, each disk has large variations in flatness, or thickness, and, due to its unique construction, cannot be made flat enough to use effectively at high speeds where the unevenness is accentuated by the speed. Second, the Flexible Diamond Product abrasive sheet is constructed from plated diamonds which have been unable to produce a smooth polished finish.
Another widely used product from 3M is the pyramid shaped Trizact abrasive which helps with hydroplaning effects. However, it is only practical for this product to be created with inexpensive abrasive media such as aluminum oxide which tends to wear fast and unevenly across its surface. Again, this is a continuous web type of product which does to have the capability of having precise thickness control.
Two common types of abrasive articles that have been utilized in polishing operations include bonded abrasives and coated abrasives. Bonded abrasives are formed by bonding abrasive particles together, typically by a molding process, to form a rigid abrasive article. Coated abrasives have a plurality of abrasive particles bonded to a backing by means of one or more binders. Coated abrasives utilized in polishing processes are typically in the form of endless belts, tapes, or rolls which are provided in the form of a cassette. Examples of commercially available polishing products include xe2x80x9cIMPERIALxe2x80x9d Microfinishing Film (hereinafter IMFF) and xe2x80x9cIMPERIALxe2x80x9d Diamond Lapping Film (hereinafter IDLF), both of which are commercially available from Minnesota Mining and Manufacturing Company, St. Paul, Minn.
Structured abrasive articles have been developed for common abrasive applications. Pieper et al., U.S. Pat. No. 5,152,917 discloses a structured abrasive article containing precisely shaped abrasive composites. These abrasive composites comprise a plurality of abrasive grains and a binder. Mucci, U.S. Pat. No. 5,107,626, discloses a method of introducing a pattern into a surface of a workpiece using a structured abrasive article.
A new class of large diameter precise thickness disks which have an annular ring of raised islands coated with a thin coat of diamond abrasive particles is required for high speed lapping which requires a completely different manufacturing technique than has been employed in the past by the abrasives industry. The new batch type of processing required to produce these disks must be practical and cost effective. Eventually, this batch process of manufacturing a disk as a separate item should be converted partially or wholly into a continuous process when product sales volume demand warrants the investment in process equipment and converting technology.
The primary competitor for the sheet fixed abrasive polishing technology is slurry lapping, which is necessarily very slow, even though it has been progressively up-dated. Slurry lapping produces a flatter surface on a workpiece at the present time than can be accomplished by high speed lapping, which has limited the sale of the high speed lapper machines. Other traditional grinding wheel machines can produce about the same flatness accuracy as the present configuration lapper but can not produce the associated smooth polish that typical workpiece parts require. Accurate flat and smooth surfaces are used to prevent leakage when parts are mated stationary with other parts or when parts are joined to dynamically rotate against each other.
High speed lapping uses expensive thin flexible abrasive coated disks which must be very precise in thickness and must also be attached to a platen that is very flat and stable. As the platen rotates very fast, this speed tends to xe2x80x9clevelxe2x80x9d the abrasive as it is presented to the workpiece surface. As only the high spots of the abrasive contact the workpiece, the remainder of the disk abrasive is not used until the high spots wear down. Thus, it is necessary for the total system to be precisely aligned and constructed of precision components to initialize the grinding. Furthermore, the wear of the abrasive must proceed uniformly across both the surface of the sheet and the surface of each island to maintain the required flatness of both the effective abrasive surface and correspondingly, the workpiece surface. These issues have all been addressed in the latest configuration of the lapper machine along with the process techniques employed in operating it. To generate even wear with rotating abrasive disks, an annular raised abrasive is used as taught in U.S. Pat. Nos. 6,120,352; 6,102,777; 6,048,254; 5,993,298; 5,967,882; and 5,910,041. However, the desired large disks are not available as the size of commercially available abrasive disks is presently limited to about 12 inches (30.48 cm) diameter. This severely limits the width of the annular ring without the resultant much slower surface grinding speed at the inside diameter of the ring. This slower speed also results in reduced material removal from the portion of the workpiece at this inside radial location. Furthermore, as the inside radial section of the abrasive disk wears slowly, the outside diameter portion progressively wears down faster which results in an uneven surface on the annular ring. Having larger nominal diameter abrasive disks with fairly narrow annular bands will inherently take care of most of these problems.
The typical workpieces that are lapped initially are not flat and have rough surfaces. Most potential customers seem to want both very flat (within 2 light bands) and smooth polished surfaces.
A preferred abrasive flat lapping process is now done in two separate steps. First, the parts are ground flat using a rigid spindle running at full 3,000 RPM speed, a very small contact force of 1 to 2 lbs. (0.454 to 0.908 kg) and typically, 3M""s metal plated diamond abrasive. Water flows between the round islands of abrasive, reducing hydroplaning. Hydroplaning typically produces a cone shaped ground surface. Second, parts are polished using a spherical action workpiece holder, with low to moderate contact forces of 2 to 15 lbs. (0.908 to 6.81 kg), and uses a smooth coated abrasive disk operating at lower speeds of about 1,000 RPM or less to prevent hydroplaning. At this time, no xe2x80x9cisland typexe2x80x9d of coated abrasive is available for polishing in combination with an effective polishing method.
Generally, use of the metal plated diamond island style abrasive disks to remove material is consider to be xe2x80x9cgrinding,xe2x80x9d as the surface finish is not smooth to the high standards of polishing. Use of the coated abrasives creates very smooth surfaces and are considered to be xe2x80x9clappingxe2x80x9d. The plated diamond disks tend to be very durable and may last a long time during use. The coated diamond and other abrasive particle disks are much more fragile and are consumed much more rapidly.
With respect to performance, with rigid flat grinding, 2 light bands of flatness are obtained which is too high for most applications. Polishing results in acceptable smoothness but typically creates new problems with flatness because of hydroplaning. Flatness defects created in the polishing step are both cone shapes and saddle shapes.
The high surface speed of the plated island abrasive creates extraordinary high rates of material removal of very hard materials and this perhaps can be increased even further with higher speeds. This is the primary reason for the interest of the high speed grinding and lapping. There probably is a significant business just in the use of this grinding portion of the process to initially prepare parts for the subsequent smooth lapping processing by other traditional methods such as slurry lapping to finish the parts. However, this initial xe2x80x9cfast grindxe2x80x9d does not appear to be of sufficient benefit to introduce this totally new technology to the marketplace.
Hydroplaning of parts using fine small particle coated abrasive will always be a problem at very high speeds until an abrasive article disk is available which has xe2x80x9cislandsxe2x80x9d of abrasive which allows excess water to pass around the island edges. A recent new commercial form of abrasive disks which has the abrasive formed into small pyramids of abrasive is available and it works well from a hydroplaning standpoint when the pyramids are fresh and to worn down. However, this Trizact brand disk sold by 3M is created only with relatively soft aluminum oxide and tends to wear out fast. It is not logical that the manufacturer would use longer wearing diamond particles in these pyramid shapes as each disk would consume so much diamond that the costs would be too high.
U.S. Pat. No. 5,611,825 (Engen) describes resin adhesive binder systems which can be used for bonding abrasive particles to web backing material, particularly urea-aldehyde binders. There is no reference made to forming or abrasive coating abrasive islands. He describes the use of make, size and super size coatings, different backing materials, the use of methyl ethyl ketone and other solvents. Loose abrasive particles are either adhered to uncured make coat binders which have been coated on a backing or abrasive particles are dispersed in a 70 percent solids resin binder and this abrasive composite is bonded to the backing. Backing materials include very flat and smooth polyester film for common use in fine grade abrasives which allow all the particles to be in one plane. Primer coatings are used on the smooth backing films to increase adhesion of the make coating. Water solvents are desired but organic solvents are necessary for resins. Fillers include calcium metasilicate, aluminum sulfate, alumina trihydrate, cryolite, magnesia, kaolin, quartz, and glass. Grinding aid fillers include cryolite, potassium fluroborate, feldspar and sulfur. Backing films include polyesters, polyolefins, polyamides, polyvinyl chloride, polyacrylates, polyacrylonitrile, polystyrene, polysulfones, polyimides, polycarbonates, cellulose acetates, polydimethyl silotanes, polyfluorocarbons. Priming of the backing to improve make coating adhesion includes a chemical primer or surface alterations such a corona treatment, UV treatment, electron beam treatment, flame treatment and scuffing. Solvents include acetone, methyl ethyl ketone, methyl t-butyl et6her, ethyl acetate, acetonitrile, tetrahydrofuran and others such as methanol, ethanol, propanol, isopropanol, 2-ethoxyethanol and 2-propoxyethanol. Abrasive filled slurry is coated by a variety of methods including knife coating, roll coating, spray coating, rotogravure coating, and like methods. Resins used include resole and novolac phenolic resins, aminoplast resins, melamine resins, epoxy resins, polyurethane resins, isocyanurate resins, urea-formaldehyde resins, isocyanurate resins and radiation-curable resins. Different examples of make, size and supersize coatings and their quantitative amounts of components were given.
U.S. Pat. No. 4,903,440 (Kirk) describes the use of different reduced-cost drum cured binder abrasive particle adhesives which allow elimination of the use of web festoon ovens which are used because of the long cure times required by conventional phenolic adhesives used for abrasive webs. Typically a pre-coat, a make coat, loose abrasive particles are imbedded into the make coat and then a size coat is applied to a continuous web backing. No reference is given to processing individual abrasive articles such as abrasive disks. Rather, a continuous backing web is coated with binders and abrasive particles, the binders are cured and then the web is converted into abrasive products such as disks or belts. Resole phenolic resins which are somewhat sensitive to water lubricants are catalyzed by alkaline catalysts and novolac phenolic resins having a source of formaldehyde to effect the cure are described. Viscosity of some binders are reduced by solvents. Fillers include calcium carbonate, calcium oxide, calcium metasilicate, aluminum sulfate, alumina trihydrate, cryolite, magnesia, kaolin, quartz and glass. Grinding aid fillers include cryolite, potassium fluroborate, feldspar and sulfur. Super size coats can use zinc stearate to prevent abrasive loading or grinding aids to enhance abrading. Coating techniques include two basic methods. The first is to provide a pre-size coat, a make coat, the initial anchoring of loose abrasive grain particles and a size coat for tenaciously holding abrasive grains to the backing. The second coating technique is to us a single-coat binder where a single-coat takes the place of the make coat/size coat combination. An ethyl cellosolve and water solvent is referenced for use with a resole phenolic resin.
U.S. Pat. No. 4,038,046 (Supkis) describes abrasive articles made with a blend of urea formaldehyde and alkaline catalyzed resole phenolic binder resins which are cured with the same curing time and temperatures as conventionally used for phenolic resins. Abrasive particles applied by gravity and also by electro-coating methods. A typical oven cure cycle of the web is 25 minutes at 125 degrees F., 25 minutes at 135 degrees F., 18 minutes at 180 degrees F., 25 minutes at 190 degrees F., 15 minutes at 225 degrees F. and 8 hours at 230 degrees F. Yellow and blue dyes are mixed in the binder system.
U.S. Pat. No. 4,710,406 (Fugier) describes a production method for the manufacture of a condensation reaction phenolic resin with different alkali catalysts and which can be diluted up to 1,000 percent.
U.S. Pat. No. 4,426,484 (Saeki) describes phenolic resins which have the cure time accelerated by suing special additives.
U.S. Pat. No. 5,304,225 (Gardziella) describes phenolic resins which typically have high viscosity which can be lowered by the addition of solvents or oils.
U.S. Pat. No. 5,397,369 (Ohishi) describes phenolic resins used in abrasive production which have excessive viscosity where a large amount of solvent is required for dilution to adjust the viscosity within an appropriate range. Examples of organic solvents with high boiling points include cyclohexanone, cyclohexanol. Solvents having an excessively high boiling point tend to remain in the adhesive binder and results in insufficient drying. When the boiling point of a solvent is too low, the solvent leaves the binder too fast and can result in defects in the abrasive coating, sometimes in the form of foamed areas. Additives such as calcium carbonate, silicone oxide, talc, etc. fillers, cryolite, potassium borofluoride, etc. grinding aids and pigment, dye, etc. colorants can be added to the second phenolic adhesive (size coat) used in the abrasive manufacture.
U.S. Pat. No. 5,674,122 (Krech) described screen abrasive articles where the abrasive particles are applied to a make coat of phenolic resin by known technique of drop coating or electrostatic coating. The make coating is then at least partially cured and a phenolic size coating is applied over the abrasive particles and both the make coat and size coat are fully cured. Make and size coats are applied by known techniques such as roll coating, spray coating, curtain coating and the like. Optionally, a super size coat can be applied over the size coat with anti-loading additive of a stearate such as zinc stearate in a concentration of about 25 percent by weight optionally along with other additives such as cryolite or other grinding aids. In addition, the abrasive coating can be applied as a slurry where the abrasive particles are dispersed in a resinous binder precursor which is applied to the backing by roll coating, spray coating, knife coating and the like. Various types of abrasive particles of aluminum oxide, ceramic aluminum oxide, heat-treated aluminum oxide, white-fused aluminum oxide, silicone carbide, alumina zirconia, diamond, ceria, cubic boron nitride, garnet and combinations of these in particle sizes ranging from 4 to 1300 micrometers can be used.
U.S. Pat. No. 4,251,408 (Hesse) describes phenolic resins used in preparation of abrasives where rapid curing as a result of increasing the curing temperature tends to form blisters which impairs the adherence of the resin to the substrate backing. Special cure cycles are used which have low initial curing temperatures with regulated, progressively increasing temperature which prevent blister formation but the time required for cross-linking is thereby increased. Drying and curing of webs by use of loop dryers or festoon dryers are discussed which provide both the function of driving off the solvents from the binder and to cross-link cure the binder. The cure rate of a resin is defined by the B-time which is the time required to change from a liquid state to reach the rubbery elastomer state (B-state).
U.S. Pat. No. 5,551,961 (Engen) describes abrasive articles made with a phenolic resin applied as a make coat used to secure abrasive particles to the backing by applying the particles while the make coat is in an uncured state, and then, the make coat is pre-cured. A size coat is added. Alternatively, a dispersion of abrasive particles in a binder is coated on the backing. The use of solvents is described to reduce the viscosity of the high viscous resins where high viscosity binders cause xe2x80x9cfloodingxe2x80x9d, i.e., excessive filling in between 30 to 50 micrometer abrasive grains. Also, non-homogenous binder resins result in visual defects and performance defects. Both flooding and non-homogenous problems can be reduced by the use of organic solvents which are minimized as much as possible. Resole phenolic resins experience condensation reactions where water is given off during cross linking when cured. These phenolics exhibit excellent toughness, dimensional stability, strength, hardness and heat resistance when cured. Fillers used include calcium sulfate, aluminum sulfate, aluminum trihydrate, cryolite, magnesium, kaolin, quartz and glass and grinding aid fillers include cryolite, potassium fluoroborate, feldspar and sulfur. Abrasive particles include fused alumina zirconia, diamond, silicone carbide, coated silicone carbide, alpha alumina-based ceramic and may be individual abrasive grains or agglomerates of individual abrasive grains. The abrasive grains may be orientated or can be applied to the backing without orientation. The preferred backing film for lapping coated abrasives is polymeric film such as polyester film and the film is primed with an ethylene acrylic acid copolymer to promote adhesion of the abrasive composite binder coating. Other backing materials include polyesters, polyolefins, polyamides, polyvinyl chloride, polyacrylates, polyacrylonitrile, polystyrene, polysulfones, polyimides, polycarbonates, cellulose acetates, polydimethyl siloxanes, polyfluocarbons, and blends of copolymers thereof, copolymers of ethylene and acrylic acid, copolymers of ethylene and vinyl acetate. Priming of the film includes surface alteration by a chemical primer, corona treatment, UV treatment, electron beam treatment, flame treatment and scuffing to increase the surface area. Solvents include those having a boiling point of 100 degrees C. or less such as acetone, methyl ethyl ketone, methyl t-butyl ether, ethyl acetate, acetonitrile, and one or more organ solvents having a boiling point of 125 degrees C. or less including methanol, ethanol, propanol, isopropanol, 2-ethoxyethanol and 2-propoxyethanol. Non-loading or load-resistant super size coatings can be used where xe2x80x9cloadingxe2x80x9d is the term used in the abrasives industry to describe the filling of spaces between the abrasive particles with swarf (the material abraded from the workpiece) and the subsequent buildup of that material. Examples of load resistant materials include metal salts of fatty acids, urea-formaldehyde resins, waxes, mineral oils, cross linked siloxanes, cross linked silicones, fluorochemicals, and combinations thereof. Preferred load resistant super size coatings contain zinc stearate or calcium stearate in a cellulose binder. In one description, the make coat precursor can be partially cured before the abrasive grains are embedded into the make coat, after which a size coating precursor is applied. A friable fused aluminum oxide can be used as a filler.
U.S. Pat. No. 5,137,542 (Buchanan) describes a coated abrasive article which has a coated layer of conductive ink applied to the surface of the article, either as a continuous film or the back side of the backing or as printed xe2x80x9cislandxe2x80x9d patterns on the abrasive particle size of the article to prevent the buildup of static electricity during use. Static shock can cause operator injury or ignite wood dust particles. The islands coated on 3M Imperial abrasive were typically quite large (1 inch (2.54 cm) diameter) dots and cover only about 22 percent of the article surface. Further, they are very thin, about 4 to 10 micrometers. No reference is made to the affect of the raised islands on hydroplaning effects when used with a water lubricant and no reference is made to high speed lapping. Raised islands of this height would provide little, if any, benefit for hydroplaning. Further, islands of this large diameter island would also develop a significant boundary layer across its surface length. Also, top coatings such as these electrically conductive particle filled materials would not allow the typically small mono layers of diamonds used in lapping films to abrasively contact the workpiece surface until the static coating was worn away, after which time it is no longer effective in static charge build-up prevention. Description is made of using polyester film as a backing material for lapping abrasive articles. Bond systems include phenolic resins and solvents include 2-butoxyethanol, toluene, isopropanol, or n-propyl acetate. Coating methods include letterpress printing, lithographic printing, gravure printing and screen printing. For gravure printing, a master rool or roll is engraved with minute wells which are filled with coatable electrically conductive ink with the excess coating fluid removed by a doctor blade. This coating fluid is then transferred to the abrasive article.
U.S. Pat. No. 5,108,463 (Buchanan) describes carbon black aggregates incorporated into a super size coat which also included kaolin.
U.S. Pat. No. 5,221,291 (Imatani) describes the use of a polyimide resin for the combination use as an adhesive bonding agent for abrasive particles, and also, to form an abrasive sheet. Diamond particles were dispersed in solvent thinned polyimide resin and coated on a flat surface with 60 micrometer diamond particles. The sheet was tested at very low speeds of 60 rpm but did remove material, leaving a smooth workpiece surface even though the particles were principally buried within the thickness of the resin which also formed the thin abrasive disk sheet. Most of the expensive diamonds are lost for use as grinding agents but the polyimide successfully bond the diamonds within the sheet.
U.S. Pat. No. 5,368,618 (Masmar) describes preparing an abrasive article in which multiple layers of abrasive particles, or grains, are minimized. Some conventional articles have as many as seven layers of particles which is grossly excessive for lapping abrasive media. He describes xe2x80x9cpartially curedxe2x80x9d resins in which the resin has begun to polymerize but which continues to be partially soluble in an appropriate solvent. Likewise, xe2x80x9cfully curedxe2x80x9d means the resin is polymerized in a solid state and is not soluble. If the viscosity of the make coat is too low, it wicks up by capillary action around and above the individual abrasive grains such that the grains are disposed below the surface of the make coat and no grains appear exposed. Phenolic resins are cured from 50 degrees to 150 degrees C. for 30 minutes to 12 hours. Fillers including cryolite, kaolin, quartz, and glass are used. Organic solvents are added to reduce viscosity. Typically 72 to 74 percent solids are used for resole phenolic resin binders. Special tests demonstrate that a partially cured resin is capable of attaching loose abrasive mineral grains which are drop coated onto test slides with the result that higher degree of cure results in lower mineral pickup and lower degree of cure results in less mineral pickup. Abrasive grains can be electrostatically projected into the make coat where the ends of each grain penetrates some distance into the depth of the make coat. No description was provided about the desirability, necessity, or ability of the grain application process having a flat uniform depth of the tops of each particle for high speed lapping.
U.S. Pat. No. 5,924,917 (Benedict) describes methods of making endless belts using an internal rotating driven system. He describes the problem of xe2x80x9cedge shellingxe2x80x9d which occurs on small width endless belts. This is the premature release of abrasive particles at the cut belt edge. He compensates for this by producing a belt edge that is very flexible and conformable. The analogy to this edge shelling occurs on circular abrasive disks also. To construct a belt, an abrasive web is first slit to the proper width by burst, or other, slitting techniques which tends to loosen the abrasive particles at the belt edge when the abrasive backing is separated at the appropriate width for a given belt. These edge particles may be weakly attached to the backing and they may also be changed in elevation so as to stickup higher than the remainder of the belt abrasive particles. Similarly, when a disk is punched out by die cutting techniques from a web section, the abrasive particles located on the outer peripheral cut edge are also weakened. This happens particularly for those discrete particles which were pushed laterally to the inside or outside of the die sizing hole by the matching die mandrel punch. Other types of cutting, slitting or punching abrasive articles from webs also create this shelling problem including water jet cutting, razor blade cutting, rotary knife slitting, and so on. Resole phenolic resins are alkaline catalyzed by catalysts such as sodium hydroxide, potassium hydroxide, organic amies or sodium carbonate and they are considered to be thermoset resins. Novolac phenolic resins are considered to be thermoplastic resins rather than thermoset resins which implies the novolac phenolics do not have the same high temperature service performance as the resole phenolics. Resole phenolic resins are the preferred resins because of their heat tolerance, relatively low moisture sensitivity, high hardness and low cost. During the coating process, make coat binder precursors are not solvent dried or polymerized cured to such a degree that it will not hold the abrasive particles. Generally, the make coat is not fully cured until the application of the size coat which saves a process step by fully curing both at the same time. Fillers include hollow or solid glass and phenolic spheroids and anti-static agents including graphite fibers, carbon black, metal oxides, such as vanadium oxide, conductive polymers, humectants are used. Abrasive material encompasses abrasive particles, agglomerates and multi-grain abrasive granules. Belts are produced by this method using a batch process. The thermosetting binder resin dries, by the release of solvents, and in some instances, partially solidified or cured before the abrasive particles are applied. The resin viscosity may be adjusted by controlling the amount of solvent (the percent solids of the resin) and/or the chemistry of the starting resin. Heat may also be applied to lower the resin viscosity, and may additionally be applied during the processes to effect better wetting of the binder precursor. However, the amount of heat should be controlled such that there is not premature solidification of the binder precursor. There must be enough binder resin present to completely wet the surface of the particles to provide an anchoring mechanism for the abrasive particles. A film backing material used is PET, polyethylene terephthalate having a thickness of 0.005 inch (0.128). Solvents used include trade designated aromatic 100 and shell CYCLO SO 53 solvent.
U.S. Pat. No. 5,318,604 (Gorsuch) describes abrasive articles made by metal plating islands of which are top coated with diamond abrasives that have been plated onto the islands. The technique employed is to create an island by printing an insulation solder photo resist insulation pattern over an electrical conducting plate and overlaying this with a woven non-electrical conduction cloth mesh. When immersed in a plating bath, a metal plated island is formed integral with the cloth mesh over the electrically exposed island areas of the photo resist covered metal conducting plate. After a minimum height of metal plated island area is built up by metal progressively covering the island area of interlocking mesh fiber strands, diamond particles are suspended in the plating bath liquid and allowed to free fall by gravity onto the mesh. Those particles that fall into the small island areas, which are very irregular in shape due to the unevenness of the interlocking fibers, are progressively plated onto the existing metal plated surfaces. The presentation of the individual particles to the raised island area is completely random. Some particles will fall deep into the xe2x80x9clog pilexe2x80x9d mesh, and others will land on the top curved surface of an individual cylindrical mesh fiber. Some of the abrasive particles will come to rest on other particles that have been plated onto the mesh, forming standing xe2x80x9crock towersxe2x80x9d. There is no possible height control mechanism which can be employed to assure that there exists a uniform flat level surface of the individual diamond abrasive particles over the complete surface area of the diamond mesh screen. Diamonds that form layer(s) below the uppermost surface of the top of the fiber xe2x80x9clogsxe2x80x9d in the xe2x80x9clog jamxe2x80x9d are not used and are wasted. Further, there is no control over the thickness variation of the woven mesh material and no description of techniques to level-smooth it down to the surface of the photo resist covered electrical conducting plate used for the plating. After sufficient plating has been achieved, the electrically insulated cloth, made of plastic fibers, is stripped away from the photoresist plate, which can be used again with another mesh cloth. The cloth can then be attached to a backing material or it can be dissolved away with strong chemicals or acids. Attaching the plated cloth with PSA (pressure sensitive adhesive) to a backing introduces new variance in the total thickness of the abrasive article. This process can be used to produce a rectangular sheet, but when a circular disk is punched out with the use of a punch-and-die set, the round surface of the die set will intersect with small portions of the typical round islands and either remove a sliver from some islands, or, leave just a sliver of a rather tall island. In either case, the shearing action of a die punch will tend to jam the sliver portion of the island into the matching die set members. This will introduce unbalanced forces that will tend to push the island, or a crescent shaped sliver of an island sideways. This will weaken the islands attachment to the disk backing. Then the problem of xe2x80x9cedge shellingxe2x80x9d described earlier occurs and these island slivers, or whole islands, will tend to break loose during grinding and scratches will occur on a lapped workpiece surface. This type of abrasive article can be used to flat grind a workpiece, but cannot be used successfully to produce a smooth polished surface. The mesh plastic cloth is used to produce the abrasive coated islands as it can be easily stripped away from the photo resist plate. Direct plating of islands of abrasive is described but is not used as it is too difficult to separate the direct plated island from the electrically exposed areas of the photo resist plate. There is no discussion of the concerns of hydroplaning of the workpiece when used at the high speeds desired for abrading with diamond which the height of the islands easily affords. Instead, there is only discussion of a passageway for the water to travel outward to flush out the swarf generated as grinding particles are removed from the workpiece surface. Gorsuch makes an attempt to produce a flat level diamond abrasive surface, indicating he is aware of only the fundamental problem with this invention. He first plates a thin layer of metal in an array of islands xe2x80x9cupside downxe2x80x9d on a smooth cylinder. Then he plates on a layer of diamonds, which is followed by adding a cloth mesh and adding a layer of metal plating on top of the diamonds which are now fully encapsulated into the thick layer of plated metal. The mesh is stripped off the drum to use the diamonds that originally lay on the flat surface of the drum. However, all the diamonds are completely buried in the plated metal and are useless for use as an abrasive article. Further, there was no description of uncurling a sheet of this material from the curvature of the drum and laying it flat for use as a disk without bending or distorting the abrasive metal plated sheet. Another part of the invention produces a disk with islands of abrasive. These are very thick disks which have a pattern of islands which are raised 25 percent to 50 percent (of the overall thickness of the disk) above the disk base or backing. A thick layer of abrasive slurry of abrasive particles mixed in a resin is deposited on a backing and the thickness is controlled by the use of mold plates. No description is made of how critical it is to control the flatness of the upper surface of the molded layer of abrasive, or of how the abrasive surface is maintained flat during wear. Further, no description was made of any of the issues of hydroplaning at high speed with water lubricants which is a primary concern for use with high speed lapping. A description is given of the use of very large hemispherical elements of metal which have a diameter of 0.5 to 3 mm which has generally only five abrasive particles which have a very large average size of 250 micrometer diameter. These abrasive particles are located at the top and along the lower side walls of each hemisphere and are metal plated to be embedded from 30 percent to 50 percent as an integral part of the metal hemisphere. These hemispheres are high enough to act as islands and the rounded tops would also aid in preventing hydroplaning at high speeds. However, this type of construction with very tall domes having only a single abrasive particle located on the very apex of the dome peak has little use for lapping. The single particle will be very aggressive in material removal but it will only produce distinct scratches as it removes a single track of material as it passes over a workpiece surface. This highest particle will have to become worn down along with some of the parent metal used for the dome construction before another particle will be active in partnership with the first. Having only five particles on a huge dome means most of the whole dome must effectively be worn down before the lower particles are engaged as grinding elements. The whole abrasive grinding load forces are so concentrated on single grains of abrasive that the grains tend to be knocked out of place, or xe2x80x9cpulledxe2x80x9d from the very strong plated metal binding. Use of expensive abrasive particles such as diamond seems totally to of place economically for this type of abrasive article construction. It has absolutely no value for lapping. None of the plating methods employed in this plating technique of forming abrasive articles has any capability of controlling the height of the particles relative to the backside of a backing, which is a critical factor for lapping at high surface speeds.
U.S. Pat. No. 4,256,467 (Gorsuch) describes an abrasive article with diamond particles plated onto an electrically insulated mesh cloth which can be cut into a xe2x80x9cdaisy wheelxe2x80x9d for use in grinding curved, convex, or concave optical lenses. A smooth metal drum is coated with an insulating resist except in circular dot areas where metal plating is desired and a material is applied to the open conducting spot areas which allows subsequent metal plating material to be separated from the drum. After some buildup of plated metal occurs at the circular spots, an electrically insulating woven cloth, typically made of common plastic fiber materials, is stretched over the drum and electroplating continues until the desired plated metal thickness is reached. Then small diamonds are suspended in the electroplating bath liquid and plating continues, trapping some of these suspended diamond particles by metal bonding them to the exposed surface of the previously plated round island areas. It is not described how these small particles migrate out of solution to the desired circular locations. Larger diamond particles will not remain in suspension and will sink to the bottom of the bath or the top only surface of the drum, depriving the bottom surface of the drum mesh of abrasives. The drum is described as being optionally rotated. After plating these diamonds on the top surface, they will all have different heights relative to the drum surface, and thus, relative to the bottom of the cloth due to a number of factors. It is well known that metal plating varies in thickness over different areas of a plated member simply due to variables inherent in an electro-plating process. Also, the woven cloth will have different thicknesses due to variations in the weaving machine performance. Also, there are variances in the thickness of individual woven cloth strands of the very fine denier fibers that are joined together to form a single strand. Further, the sleeve of material is stretched and pulled over the cylindrical drum, which can cause variations in the cloth thickness around the surface of the drum. All of these factors result in a flexible abrasive article which can be cut into weak strips fanned out from a common hub which will conform to a curved lense when used at very low speeds, but not for high speed lapping which requires extremely precise abrasive article thickness control. Again, in this patent, as was the case in his U.S. Pat. No. 5,318,604, he acknowledges and addresses the issue of obtaining an abrasive article that does, in fact, have all the abrasive particles in the same plane. This is done producing a cloth mesh island abrasive covered article with use of plastic cloth over a patterned drum. Here, he electroplates islands of metal over exposed areas and electroplates particles dropping out of the plating solution to these plated islands after which he continues to build up the metal plating thickness, add a cloth, continue plating, and then remove the cloth mesh from the drum. The resultant article would seem to have little use as a abrasive article as the diamond particles are not exposed at the drum surface, but rather, are enclosed or buried within the plated metal layer by the progressively built-up plating metal. As they are not exposed from the plated metal surface, they cannot effect their abrasive cutting action. Also, the backside thickness of plated metal would vary in height due to variances in the deposition rate of material over each island site to variances in electrical conductivity of the unknown coating applied over each site which allows the plated metal to be peeled from the drum. When the cloth is turned over, and mounted to a backing, the variance in height of each island, as measured from the front surface of the diamonds to the cloth bonded surface of the backing, will be significant over the whole surface of the abrasive article. This abrasive article would have no use for high speed lapping where the high speed of a rotating platen establishes an abrasive sheet mounting flatness plane more precise as the platen rotation speed is increased. The requirements of high speed lapping far exceed the capability of this system of creating abrasive articles.
U.S. Pat. No. 5,549,962 (Holms) describes the use of pyramid shaped abrasive particles by use of a production tool having three-dimensional pyramid shapes generated over its surface which are filled with abrasive particles mixed in a binder. This abrasive slurry is introduced into the pyramid cavity wells and partially cured within the cavity to sufficiently take on the shape of the cavity geometry. Then the pyramids are either removed from the rotating drum production tool for subsequent coating on a backing to produce abrasive articles, or, a web backing is brought into running contact with the drum to attach the pyramids directly to the backing to form an abrasive web article. If a web backing is used is contact with the drum, the apexes of the pyramids are directed away from the backing. If loose discrete pyramids are produced by the drum system, the pyramids can be oriented on a backing with the possibility of having the pyramid apex up, or down or sideways relative to the backing. The pyramid wells may be incorporated into a belt and also, these forms can extend through the thickness of the belt to aid in separating the abrasive pyramid particles from the belt.
Over time, many attempts have been made to distribute abrasive grits on the backing in such a method that a higher percentage of the abrasive grits can be used. Merely depositing a thick layer of abrasive grits on the backing will not solve the problem, because grits lying below the topmost grits are not likely to be used. The use of agglomerates having random shapes where abrasive particles are bound together by means of a binder are difficult to predictably control the quantity of abrasive grits that come into contact with the surface of a workpiece. For this reason, the precisely shaped (pyramid) abrasive agglomerates are prepared. Some pyramid-shaped particles are formed which do not contain any abrasive particles and these are used as dilutants to act as spacers between the pyramid abrasive agglomerates when coated by conventional means. Many different fillers and additives can be used including talc and montmorillonite clays. Care is exercised to provide sufficient curing of the agglomerate binders in the drum cavities so that the geometry of the cavity is replicated. Generally, this requires a fairly slow rotation of the production tooling cavity drum. No description is given to the accuracy of the height or thickness control of the resultant abrasive article which incorporates these very large agglomerate pyramids which typically are 530 micrometers high and have a 530 micrometer base length. Thickness variations of conventional lapping disk abrasive sheets generally are held within 3 micrometers in order for it to be used successfully. The system of using the large pyramids described here cannot produce an abrasive article of the precise thickness control required for high speed lapping for a number of fundamental reasons. Some of these reasons are listed here. First, creation of many precise sized pyramid cavities by use of a belt that is replicated into a plastic form to control the belt cost adds error due to the sequential steps taken in the replication process. Variations in binder cures from production run to run and also variations in binder cures across the surface of a drum belt result in pyramids that are distorted from the original drum wells. For backing belts to be integrally bonded to the pyramids during the formation of the pyramids, it is required that any adhesive binder used to join the agglomerate be precisely controlled in thickness. This is difficult to achieve with this type of production equipment as there are many process variables which must be controlled in addition to those control variables used to successfully create precise pyramids. The backing material must be of a precise thickness. Random orientation of the large agglomerates will inherently produce different heights at the exposed tops of the agglomerates depending on whether an agglomerate has its apex up, it is lays sideways, or has its sharp apex embedded in a make coat of binder. The use of pyramids where all the apexes are up and the bases are nested close together produces grinding effects that change drastically from the initial use where only the tips of the pyramids contact the workpiece, to a final situation where the broad bases contact the workpiece when most of the pyramid has worn away. There was no description of the inherent advantage of the use of upright pyramids for hydroplaning or swarf removal which is a natural affect of these relatively tall xe2x80x9cmountain pyramidsxe2x80x9d and the xe2x80x9cvalleysxe2x80x9d between them which can carry off the water quite well. There was no discussion of the use of this pyramid material for high speed lapping or grinding. The water lubricant effects on grinding would change significantly as the abrasive article wears down. There is a fundamental flaw in the design of the pyramid for upright use. Most of the abrasive material contained on the pyramid lies at the base which is worn out last during the phase of wear when the variations in thickness of the backing, and other thickness variation sources, prevent a good proportion of the bases from contacting a workpiece surface. When using these large-sized pyramid agglomerates, they are designed to progressively breakdown and expose new cutting edges as the old worn individual abrasive particles are expended as the support binder is worn down, exposing fresh new sharp abrasive particles. Most of the value of the expensive abrasive particles lies in the base, as most of the volume of a triangle is in the base. Here, most of the valuable abrasive particles at the base areas will never be used and are wasted. Further, as wear-down of the pyramids is prescribed by selection of the pyramid agglomerate binder, the level surface of the abrasive disk will vary from the inside radius to the outside radius as the contact surface speed with a workpiece will be different due to the radius affect of a rotating abrasive platen. The pyramids are grossly high and uneven wear far in excess of that allowable for high speed lapping prevents the use of this type of article for high speed lapping. Inexpensive abrasive materials such as aluminum oxide can be used for the pyramid agglomerates but it is totally impractical to use the extra hard, but very expensive, diamond abrasives in these agglomerates. The flaws inherent in the use of conventional agglomerates due to size variations in the agglomerates does not make any sense. First, agglomerates can be made and then sorted by size prior to use as a coated abrasive. Also, the configuration of a generally round shaped conventional agglomerate would certainly wear more uniformly than wearing down a pyramid which has a very narrow spiked top and, after wear-down, a base which is probably ten times more large in cross-sectional surface area than the pyramid top. Random orientation of the pyramid shape does not help this geometric artifact. Another issue is the formulation of the binder and filling used in a conventional agglomerate. A wide range of friable materials such as wood products can be joined in a binder which can be selected to produce an agglomerate by many methods, including furnace baking, etc. The binder used in the production of the pyramids must be primarily selected for process compatibility with the fast cure replication of the drum wells and not for consideration of whether this binder will break down at the desired rate to expose new abrasives at the same rate the abrasive particles themselves are wearing down. It does not appear that this pyramid shaped agglomerate particle has much use for high speed lapping. Use of a polyethylene terephthalete polyester film with a acrylic acid prime coat is described.
U.S. Pat. No. 4,799,939 (Bloecher) describes use of 70 micrometer diameter hollow glass spheres which are mixed with abrasive particles and a binder to form erodible 150 to 3000 micrometer agglomerates which are used for coating in abrasive articles. The hollow glass spheres are strong enough for the mixing operation and for the process used to form the agglomerate particle. However, they are weak enough that they break when used in grinding. Again, as for U.S. Pat. No. 4,652,275, these agglomerates are much too large and inappropriate for use in high speed lapping.
U.S. Pat. No. 4,327,156 (Dillon) describes a plastic mold cavity made from a powdered metal binder mixture that was molded in a RTV rubber mold. An A-6 tool steel powder is mixed with a thermosetting adhesive binder that is diluted with a liquid that is a good solvent for the uncured binder but poor solvent for the fluid binder. This diluent/thermoset binder can be mixed with powdered metals, deposited in a mold, solidified by curing and the form shape can be fired in a furnace to produce an exact replica of the original mold shape that is a few percents smaller than the original shape. The diluent comes out of phase with the thermoset binder and is exhausted from the green powder shape, leaving the thermoset binder attaching each powdered metal particle bound to adjacent particles. Furnace heating is continued at a higher temperature and a porous metal shape is created which can be filled with molten copper by wicking action. Here, a completely solid metal form has been produced which is an extremely accurate representation of the original shape. This same technology can be used to form island base foundations of raised abrasive islands.
These systems have been described as providing benefits to particular technical and commercial fields, but they have not been shown to provide any particular benefits to truly high speed lapping/polishing systems and materials. No operational speeds are listed in any of the reference patents listed here indicates a lack of interest or awareness of the resultant artifacts of high speed lapping or polishing.
U.S. Pat. No. 4,652,275 (Bloecher) describes the use of erodible agglomerates of abrasive particles used for coated abrasive articles. The matrix material, joined together with the abrasive particles, erodes away during grinding which allows sloughing off of spent abrasive particles and the exposure of new abrasive grains. The matrix material is generally a wood product such as wood flour selected from pulp. A binder can include a variety of materials including phenolics. It is important that the binder not soften due to heat generated by grinding action. Instead, it should be brittle so as to breakaway. If too much binder is used, the agglomerate will not erode and if too little is used, the mixture of the matrix and the abrasive particles are hard to mix. The preferred agglomerate is made by coating a layer of the mixture, curing it, breaking it into pieces and separating the agglomerate particles by size for coating use. Agglomerates of a uniform size can be made in a pelletizer by spraying or dropping resin into a mill containing the abrasive mineral/matrix mixture. Agglomerates are typically irregular in shape, but they can be formed into spheres, spheroids, ellipsoids, pellets, rods and other conventional shapes. Other methods of making agglomerates include the creation of hollow shells of abrasive particles where the shell breaks down with grinding use to continually expose new abrasive particles. Other solid agglomerates of abrasive particles are mixed with an inorganic, brittle cryolite matrix. A description is made of conventional coated abrasives which typically consist of a single layer of abrasive grain adhered to a backing. It has been found that only up to 15 percent of the grains in the layer are actually utilized in removing any of the workpiece. It follows then that about 85 percent of the grains in the layer are wasted. The agglomerates described here preferably range from 150 micrometers to 3000 micrometers and have between 10 and 1000 individual abrasive grain particles for P180 grains and only 2 to 20 grains of larger P36 grains. These agglomerates far exceed the size required for high speed lapping. In fact, only single layers of diamond particles is required or typically used as a coating for most lapping abrasive articles, so these huge agglomerates have little or no use in lapping. Further, there would not be an effective method of maintaining a flat abrasive surface as the abrasive agglomerates are worn down by abrasive lapping or grinding action.
Lapping or grinding with abrasives fixed to a flexible sheet is operated at high surface speeds of 10,000 surface feet per minute, requiring the use of water-like lubricants to cool the workpiece and to carry away grinding swarf. A workpiece can be held rigidly or flexibly by a rotating spindle to effect grinding contact with a rotating abrasive platen, but the spindle must be maintained precisely perpendicular to the abrasive surface to obtain a workpiece surface flat within about 2 lightbands. The aggressive cutting action of plated diamond island style flexible sheets requires the grinding contact perpendicular force to be near zero pounds at the start and end of the grinding procedure and to be controlled within plus or minus 0.5 pounds (227 grams) with a typical nominal force of 2.0 lbs. (0.908 kg) for an annular ring shaped workpiece having approximately 3.0 square inches (58.1 square cm) of surface area. Hydroplaning of the workpiece on the water lubricated abrasive is minimized when using abrasive covered raised island sheets, but is severe for uniformly coated abrasive disks generally used for smooth polishing or lapping. Hydroplaning causes cone shaped ground workpiece surfaces, even with raised platen annular rings. Lapping requires a low friction spherical action workpiece holder which does to tilt due to abrasive contact forces. A lightweight three-legged offset center-of-rotation spherical workholder with fluid joints and a link arm connecting the two matching spherical mechanism segments can significantly reduce workpiece tilting due to abrasive planar contact forces. Rotating the workpiece holder in the same clockwise or counter clockwise direction as the abrasive when using large diameter disks with narrow annular bands of thin coated raised abrasive islands is an effective method to flat-grind or lap-polish workpieces with increasingly large diameters. The abrasive platen must be ground very flat and the abrasive disk sheet must be precise in thickness to be used effectively at high speeds.
Abrasive disks of large 18 inch (0.457 m), 24 inch (0.609 m), 36 inch (0.914), 48 inch (1.22 m) or even 60 inch (2.3 m) diameter having an outer annular band of raised islands which have a thin precise coating of diamond particles can be produced effectively with very precise thickness control. Raised islands can be deposited by a variety of means on a variety of commonly available thin flexible plastic or metal backing materials. Loose diamonds can be metal plated or plastic binder coated as a single mono layer on top of these islands which have been height controlled to produce a precisely controlled thickness relative to the bottom surface of the disk backing material. Diamond particles can be coated with the use of binders such as phenolics which have been used traditionally in the abrasive industry for many years. A make binder coating can be applied to a backing material, abrasive particle powder applied, a partial or full cure effected and a filled size coat applied and then the full substrate disk cure effected. These disks principally would be produced by a batch process, but the basic process can also be applied to continuous webs. Fine abrasive particle disk sheets or belts can be used for lapping and coarse particle disks used for grinding.